A typical, optically pumped, fluid cooled, slab laser includes a slab of solid-state lasing medium. Pumping means such as a lamp and reflector are situated so as to impinge optical radiation on the lasing medium whereby to pump the atoms in the lasing medium to a metastable state. A cooling fluid is flowed across at least a portion of the lasing medium to remove heat generated therein by the optical pumping. A laser beam is subsequently produced by passing a beam of coherent light, either externally generated or stimulated within the lasing medium, oscillatingly through the lasing medium. This coherent light beam is amplified with each pass through the lasing medium.
Such slab lasers typically exhibit both width-wise and thickness-wise wave-front distortions of the laser beam, the width-wise distortion being particularly prominent in areas proximate lateral edges of the lasing medium. This wave-front distortion diminishes the usable area of the lasing medium, the efficiency of the slab laser, and the quality of the laser beam. This wave-front distortion is caused by at least three known phenomena, including: (1) width-wise variation of the refractive index of the lasing medium caused by thermal gradients within the lasing medium, i.e. thermal lensing; (2) variations in the refractive index of the lasing medium due to a stress-optic effect initiated by the flow of cooling fluid over the lasing medium; and (3) beam distortion induced by a deformation of major faces of the lasing medium, also caused by the cooling fluid flow over the lasing medium.
U.S. Pat. No. 3,633,126 (Martin et al.), incorporated herein by reference, addresses the problem of thickness-wise variation of the refractive index of the lasing medium. In Martin et al., the beam of coherent light is introduced into the lasing medium in an off-axial direction, such that each ray in the beam is multiply, internally reflected through regions of varying temperatures and refractive indexes during each pass through the lasing medium. These varying refractive indexes in the various lasing medium regions are thereby averaged across the beam, minimizing their distortional affects on the beam. Martin et al., however, does not address width-wise wave-front distortions within the lasing medium.
Several other methods and apparatus have been utilized to correct wave-front distortion in slab lasers. Optical phase conjugation has been found suitable for use with Q-switched, high peak power lasers. However, it is not adaptable to industrial lasers which operate with long pulse widths or in a continuous-wave mode. Adjustment of coolant flow over the edges of the lasing medium slab provides some control over edge distortion, but produces increased stress and thus increased breakage proximate the edges of the slab. Correcting lenses disposed external to the lasing medium can be used to provide some correction for wave-front distortion, but only to uniform spherical distortion and only at one operating power.
Thus, wave-front distortion caused by stress-optic effects and thermal lensing has not been adequately addressed by the prior art. As stated above, these distortions are most dominant proximate lateral edges of the lasing medium, thereby causing users of these slab lasers to avoid utilizing the edges of the lasing medium to generate the laser beam. Typical active lasing mediums, for example slabs of YAG (yttrium aluminum garnet) crystals, are limited in size by current crystal growth technology. Thus, it would provide a substantial benefit to the art if a slab laser could be devised wherein wave-front distortion does not prohibit the full width of the lasing medium from being utilized to generate the laser beam.